As a common antiskid means for an automobile tire used on a frozen road, metal spikes used to be popularly mounted to the tire. According to this means however, when the automobile is running, the spikes grind the road surface so excessively that the road is severely damaged. In addition, a large amount of dust results. To overcome these problems there is proposed a means such as one disclosed in Japanese Patent Publication No. 1-249003.
According to the means disclosed in the above patent publication, pins made of a predetermined rubber component are mounted to the tire instead of the metal spikes. The pins harden at and below the freezing temperature whereas they soften at higher temperatures. Such a means makes possible to use the hardened pins as spikes at temperatures below the freezing point when the road is frozen. On the other hand, at normal temperatures when the road is not frozen, the pins are softened so that it becomes possible to prevent the pins from unduly grinding the road surface.
However, the above prior art has the following disadvantages.
First, when the car is running, the tires of the car generate eat due to intermolecular friction of the tire rubber and friction from the road, and the heat transfers to each of the pins. Specifically, when the car is running at 40 km/h, the rubber of the tire has an internal temperature of 60.degree. C. to 70.degree. C., increasing the temperature of the pins. Thus, according to the above prior art, even if the car is running on a frozen road, there is an occasion when the heat from the tire prohibits the pins from hardening to a sufficient level necessary for functioning as the spikes.
This problem may be solved by making the pins harden at a higher temperature. However, such can only be achieved by mixing the material rubber with a large amount of additives, which decreases elasticity and strength of the pin, leading to another problem that the pins would be easily damaged when contacted by the road.
Second, when the car is running, the tip potion of the pin contacts the frozen road of a low temperature whereas the base portion of the pin receives heat from the tire. As a result, there is a large temperature difference between the tip portion and the base portion of the pin, causing a substantial difference in the hardness between the tip portion and the base portion of the pin. The base portion which is softer becomes more susceptible to deformation. This leads to a problem that stresses from the operating tire concentrate on the base portion of the pin, causing the pin to fail from fatigue.